Electrostatic discharge (ESD) is the sudden flow of electricity between two objects caused by a contact, an electrical short, or a dielectric breakdown. ESD can be caused by a buildup of static electricity by tribocharging, or by electrostatic induction. ESD includes spectacular electric sparks, but also less dramatic forms which may be neither seen nor heard, yet large enough to cause damage to sensitive electronic devices. ESD can cause a range of harmful effects, as well as failure of an integrated circuit (IC).
An electrostatic discharge (“ESD”) protection circuit may be needed for the IC. The ESD protection circuit provides a path to bypass current from a terminal of the IC to a ground or from the terminal of the IC to a power supply rail, so that the current due to an ESD event can bypass the internal circuitry of the IC. Voltages far in excess of the normal operating voltages, in both positive and negative magnitudes, are observed during short duration electrostatic discharge events. Therefore, the ESD protection circuit prevents the ESD current from destroying sensitive components in the IC.
The ESD protection circuit can be triggered or activated in response to a trigger voltage over a threshold. Snapback devices such as silicon controlled rectifier (SCR) circuits and bipolar junction transistors (BJTs) may be used as ESD protection circuits. However, an SCR circuit has potential burn out risks caused by mistakenly triggered ESD events since the trigger voltage of an SCR circuit may change when ESD events generate waveforms having different rising times.
After triggering or activating the ESD protection circuit, the ESD protection circuit remains active as long as a voltage is over a “holding voltage.” If the “holding voltage” of the ESD protection circuit is too low, the ESD protection circuit may likely stay active when the IC is no longer under ESD stress. Accordingly high holding voltages for ESD protection circuits are desirable. The holding voltage of a snapback device including at least one SCR circuit an at least one BJT may be determined by the holding voltage of the SCR circuit. The holding voltage of the SCR circuit is low in most cases, thus leading to a snapback device with a low holding voltage. The low holding voltage leads to large gap between the trigger voltage and the holding voltage of these snapback devices. Improved ESD protection circuits are needed with high holding voltages, and reduced gap between the trigger voltage and the holding voltage, which is more suitable for high voltage applications.